The present invention relates to a device for generating an optically homogeneous streak-free quartz glass body having a large diameter. In accordance with the invention, bodies made of synthetic quartz glass and having a high homogeneity of their optical properties such as refractive index distribution, transmission, and fluorescence can be manufactured, in particular for use with wavelength under 250 nm.
The transmission and radiation stability required for an application in the UV-range, for example, in excimer lasers or in the microlithography at wavelengths below 250 nm, demands an extremely high purity of the manufactured quartz glass bodies which generally is ensured by a procedure via the gas-phase. In a flame hydrolysis method, a gaseous Si-starting-compound together with oxygen and a fuel gas is converted in a flame to yield SiO2-particles which are vitreously molten to the surface of a quartz glass body heated by a burner.
The increasing demands in the field of microlithography to increase the numerical aperture at a simultaneously high homogeneity of the optical components resulted in the manufacture of larger and larger quartz glass blanks. The quartz glass cylinders conventionally produced by non-displaceably fixing a single burner relative to a surface on which the quartz glass cylinders are molten are, however, limited with respect to the maximum diameters that can be attained. A transformation of the glass cylinders obtainable by this method into disks of expensive and involving the possibility of a negative variation of material, which unfavorably affects transmission and laser stability. Thus, a widening of the deposition range is required for a direct deposition on the desired final format as it is obtained by the conventional use of a plurality of burners or by moving a burner relative to the glass body in a plane normal to the axis of the quartz glass body or by a combination of both methods.
A particularly demanding technological request is to satisfy the requirements for a considerably larger area, which requirements the quartz glass body has to satisfy as they relate to the homogeneity of the optical properties even with larger diameters. This particularly is applicable to the homogeneity of the refractive index distribution with the currently used wavelengths of the mercury vapor lamp (365 nm-i-line and 436 nm-G-line) in microlithography.
Below 250 nm, however, particular consideration has to be given to a constant high transmission to be maintained over the entire optically usable area as well as an excellent damage behavior to be ensured over this area. When these short wavelengths are used, then the irradiation takes place already in the vicinity of the band edge of the glass. Hence, the smallest variations of the structure and the concentration in the glass arising from the manufacture take effect on the glass. Such variations lead to a displacement of the band edge and hence to the transmission below 200 nm with transmission variations in an order of size of some 0.001 cmxe2x88x921 and more (given as decimal absorption coefficient) which is as it relates to microlithography of relevance. As a result of the light scattering, increasing with xcex4, the diffusion rate for quartz glass, depending on the specific glass structure, lies also in an order of size of some 0.001 cmxe2x88x921.
Additionally, there are numerous intrinsic defects in synthetic glass known, such as Cl-inclusions, OH-groups, oxygen defects etc., having absorption bands in a range between 150 nm and 250 nm. In view these strong dependencies of the transmission below 250 nm from the material properties, significant local transmission variations within a quartz glass body will arise with the conventional manufacturing methods.
Japanese Patent No. 01-024032 describes the deposition of quartz glass in a rotating cylindrical vessel wherein a burner performs a rotating or linear movement to feed a gaseous Si compound as well as the fuel gases. In a further embodiment, a second burner, which exclusively is gas-fueled, is used as an auxiliary heater. The deposition of SiO2 particles in a vessel unavoidably leads to strong variations of the flow conditions which, in turn, lead to variations in the deposition conditions during the process. As a result, there are intense streak formations substantially at right angles to the deposition direction.
In Japanese Patent No. 06-234531, a burner providing O2-compounds, H2-compounds, and an Si-compound is moved in a plane relative to a quartz glass ingot being constructed in accordance with the temperature distribution measured at the ingot head. The temperature distribution detected by an IR- camera is fed into a computer adapted to control an x,y-table, upon which the ingot is mounted.
EP 0 735 006 describes the movement of a quartz glass ingot in the x-direction and y-direction at right angles to the deposition direction of the synthetic quartz glass. The resulting quartz glass contains streaks which substantially lie at right angles to the deposition direction.
U.S. Pat. No. 5,696,038 describes a relative and oscillating motion between a source of SiO2-particles and the quartz glass body being constructed, the motion taking place at right angles to the width of the body at a definite periodicity. There are described in detail cycle periods, which allow a definite minimum layer thickness deposition per cycle, which have a definite minimum duration, and show helically shaped paths or which limit the formation of streaks. The deposition of SiO2 is performed in a flat vessel which is heated from top by a plurality of burners fixed in the upper part of a furnace. The manufactured material contains streaks at right angles to the deposition direction.
European Patent Publication No. EP 0 850 199 relates to those problems caused by the air streams which result from the motion of quartz glass body in a configuration according to U.S. Pat. No. 5,696,038. A method and device is claimed for maintaining a constant air stream about the growing quartz body while the latter performs oscillating movements. The deposition of SiO2 is performed in a flat vessel in analogy to U.S. Pat. No. 5,696,038, which is heated from top by a plurality of non-displaceable burners. The manufactured material also contains streaks at right angles to the deposition direction.
Moreover, with all described prior art solutions, there is, due to the unfavorable flow conditions, the danger of introducing microstructural defects or glass bubbles in the material, apart from streaks.
European Patent Publication No. EP 0 720 970 discloses a quartz glass having a structure temperature of less than or equal to 1200 K, an OH-content of at least 1000 ppm, and an internal transmission of at least 99.6% at 193 nm. However, it does not discuss the steadiness of the transmission over the face of a quartz glass body.
U.S. Pat. No. 5,696,624 describes protection for a quartz glass of an internal transmission of greater than 99.9% per 1 cm sample thickness across a diameter of at least 150 mm before and after bombardment with 106 pulses of an ArF-laser. It is well-known that the internal transmission in the UV range can only be exactly measured by large expenditures. Any kind of surface contamination and surface defects (for example, by polishing) results in a severe reduction of the UV transmission that with highly transparent material makes up a multifold of the measuring effect of the internal transmission. For this reason, the purification of the samples gets a predominant significance. There is no reference to that problem in the above-cited publication. Generally, surface effects can be eliminated by carrying out thickness-dependent series of measurements.
In the measuring method described in U.S. Pat. No. 5,696,624 two samples having lengths of 2 mm and 12 mm were used. There is used a dual-beam spectrophotometer, the publication has, however, no reference to a purging of the spectrometer with a purging gas being non-absorbing in the UV-range, for example, N2, this being necessary according to the prior art. This not being carried out will lead to a falsification of the transmission towards higher values, due to the UV absorption by oxygen. The determination of the transmission at 193 nm is related to that at 365 nm, the value of which is arbitrarily set to 100% for the internal transmission. Such a measuring is obviously improper, since the given internal transmissions of greater than 99.9% cmxe2x88x921 are physically not reasonable.
In U.S. Pat. No. 5,696,624, the internal transmission is, as generally used, defined as the spectral transmission measured minus the reflection losses at the surface. As generally known, the dispersion losses in the UV, which increase with the fourth power of the wavelength, significantly contribute to the losses of internal transmission. So, for example, a dispersion loss of the transmission of at least 0.18% related to one centimeter is given in the already mentioned EP 0 720 970; Lieberman et al, in Opt. Lett., Vol. 24 (1999), pg. 58-60 states a decimal volume dispersion coefficient of 0.0008 cmxe2x88x921, which also corresponds to a transmission loss of 0.18% related to one centimeter. Alone by this dispersion amount, even without additional absorption rates from the range of the absorption edge or from intrinsic defects which, as explained above, add a not insignificant amount at the wavelengths considered, a transmission of 100% is exceeded in a physically non-reasonable manner.
In International Publication No. WO 98/40319, a deposition melting device, working according to a horizontal method, is described, by which a good optical homogeneity, a high transmission, a low fluorescence, and a high laser stability can be achieved. This deposition melting device substantially has a rotation symmetrical design, exhibits a favorable distance between its interior shape and the cap of the quartz glass body to be deposited by melting, and comprises a geometry which favorably effects the heat economy in the device.
The diameter of the manufactured streak-free pre-forms is, however, comparatively small, so that the pre-form has to be work-expensively thermally transformed in order to obtain larger final products.
Therefore, it is an object of the present invention to provide an arrangement which allows production of pre-forms from streak-free quartz glass having large diameters (preferably  greater than 180 mm), whereby the quartz glass over its entire diameter exhibits a high homogeneity of is the optical properties (refractive index distribution, transmission, fluorescence), but no micro-structural defects. Streaks or striations are here referred to as all local variations of the refractive index in an order of size that can be visually detected and by means of the shadow method, respectively, or in a similar direct manner. The term xe2x80x9cstreak freexe2x80x9d refers to streaks or striations extending in at least one spatial direction within the glass, preferably, in parallel to the optical axis of the quartz glass body, whereby the optical axis is centrally extending in the quartz glass body in the deposition direction of the same.
According to the invention, the object is realized by an apparatus for generating optically homogeneous streak-free quartz glass bodies having large diameters which comprises a melting device having an inner space provided with first and second opposing openings, at least one burner arranged in the first opening, a quartz glass body having an axis X-X displaceably arranged in the second opening and means capable of providing relative movement between the burner(s) and the quartz glass body in an axial direction and in a radial direction such that the distance between the burner(s) and the quartz glass body is reduced with an increasing distance of the burner(s) from the X-X axis of the quartz glass body during generation of an optically homogeneous streak-free body.
The invention allows manufacture of pre-forms having large diameters from synthetic quartz glass by the flame-hydrolysis method, whereby the quartz glass body over its entire diameter exhibits, with the exception of a small rim portion, a very good refractive index homogeneity and a uniformly high transmission.
According to the invention both, the burner and the quartz glass body can be involved in the relative motion between the two. When the relative motion is carried out only by the burner, then the interior shape of the melting device (muffle) can be very easily adapted to the desired shape of the glass. In the easiest case, the reduction of the distance between the burner exit opening (SiO2-source) and the increasing diameter of the pre-form takes place linearly to the radial distance of the point of impingement of the flame from the axis of rotation of the pre-form. The final value of the radial distance in the rim-portion shall not be smaller than 65% of the central near-axial is distance. In this manner, the different thermal conditions and deposition conditions in the rim portion of the developing quartz glass body are taken into account. These differences are attended to by way of respective different procedure defaults with respect to the interior range and the exterior range of the quartz glass body. For this, the distance reduction between the burner exit opening and the glass body is decisive.
The following investigations have been carried out in the course of working out the arrangements in accordance with the invention:
measuring of the field of speed in the interior of the flame by way of laser-Doppler-anemometry,
measuring of the temperature field in the interior of the flame by way of Raman-spectroscopy,
measuring of the particle distribution in the interior of the flame by way of Rayleigh-radiation.
It was found that both the absolute values and the distribution of these measured variables are varying in a wide range as a function of the distance between the burner exit opening and the deposition surface and therefore can be utilized in a specific checking and control of the deposition process. Moreover, information, obtained from the Raman measurement and from chemical balancing computations, about the local chemical compositions in the burner flame open up the possibility of a correspondingly favorable setting of the concentrations of the intrinsic contaminations, for example, by Sixe2x80x94OH-groups or Sixe2x80x94Cl-groups etc. In this manner, unfavorable temperature distributions and flow conditions and, hence, the formation of streaks or bubbles are avoided across the entire cross-section having a diameter or an edge length of at least 15 cm.
The high optical homogeneity and transmission in the external range or in the rim portions of the quartz glass body will be favorably influenced when the relative motion between the burner and the glass body is accompanied by a setting of the angle and by varying the angle, respectively, between the axis of the burner and the axis of the glass body. The two axes are inclined towards each other in the deposition device in direction of the ejected fuel gas, without the necessity of having an intersection point with each other. That is, they also can be obliquely directed relative to each other and must not have common plane. In the case of strongly sloping rim portions of the quartz glass body, only then when there is an angular position of the burner axis relative to the axis of the quartz glass body, the reduction of the distance between the burner and the glass body to the required measure can be realized. The mentioned variation of the angle preferably will lie between 0xc2x0 and 60xc2x0.
In a preferred embodiment according to the present invention, the angle between the axis of the burner and the optical axis of the quartz glass body will increase uniformly monotonous with an increasing distance of the point of impingement of the flame from the axis of the quartz glass body. It contributes to the realization of the object of the invention, when there is a rotational movement performed by the preferably cylindrical perform, and when this rotational movement is integrated into the path of motion of the burner. In this manner, the expenditures for displacement units and control mechanism can be reduced. A correspondingly designed link motion device can be used to control the path of motion of the burner.
It is, however, advantageous when the control is performed with the aid of a robot or by a cross-slide system controlled by a computer. In this manner, a motion of the burner along three coordinates is ensured. Additionally, the burner is displaceably seated on the cross-slide system and at the robot lever, respectively, in parallel to its axis. In order to avoid undesired heat losses during the melting process, a displaceable stop is attached opposite the muffle for closing the associated opening of the muffle. It is also possible to provide a plurality of such stops in the muffle opening, which can be controlled simultaneously, simultaneously and/or independently from each other.
In view of diffusion, there will develop significant variations in concentration of the mentioned contaminations in the cooling phase of the manufactured quartz glass body and in a subsequent annealing process, if required. For this reason, a concentration distribution is preferably to be kept to across the melting area, in which subsequent concentration variations, due to diffusion, have already been included, so that a substantially uniform distribution will be attained for the final product. It was found by the investigations carried out with regard to the invention that, in order to render the optical properties homogeneous in the developing quartz glass body, there is required a special control of the deposition conditions including a defined insertion of desired structural effects by way of an adaptation of the temperature conditions and the flow conditions as well as of the stoichiometry of the flame in the impinging area of the flame. In order to simultaneously optimize all these parameters, the arrangement according to the present invention is particularly suited by moving the burner (SiO2-source) in direction of the axis of the quartz glass body, which preferably has a cylindrical shape, in dependence on the radial position of the burner relative to the quartz glass body.
A quartz glass body manufactured in accordance with the arrangement as of the present invention has a refractive index homogeneity of  less than xc2x11 ppm over at least 50% of its diameter, which is 180 to 500 mm. It exhibits an internal transmission of at least 0.995 at a wavelength of xc3xa=193 nm related to a width of 1 cm.